Slitherine

I found most of the time there is not many relation between fog and fog empire , as for number as for troop quality.
also there is no evolution of quality unit with time.
a new nation is create, she could have elite unit. your army fight since 10 years but your units are still under average, not fun

Empires unit Experience does translate to upgrading the quality of the FOG2 units.

However, unit quality is also affected by other factors, such as the unit's current Empires effectiveness rating, and the relative skill ratings of the opposing generals.

These may cancel out the effect of high Experience if the unit is currently at a low Effectiveness rating, or your general is outmatched.